How to make texture repeat in X but clamp in Y?
1. General solution (doesn't work in your case)
For many purposes, you can just change the texture repeat settings on the image texture node.

2. More complex solution (works perfectly in this case)
For more complex cases like yours, however, you this can be done quite easily with a mapping node:

Set the min value of, e.g., the y-axis to be whatever works best for your image. The setting 0.4 seems to work well for my simple testing setup shown below.